1. Field of the Disclosure
This disclosure relates to receiving audio/video content.
2. Description of Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
As background technology, the DVB Common Interface (“CI”) specification allowed a television receiver or set top box (a “host”) to interact with a secure hardware module (a conditional access module or “CAM”) to allow the host to decrypt access-controlled audio/video (audio, video or audio and video) content. The CI specification defines an interface between the host and the CAM, so that the two will work together if both conform to the CI specification. This interoperability provided a significant benefit of the CI system, as, in principle, it allowed consumers a choice of compatible products from different manufacturers.
In the CI specification the CAM interacts with a smart card and/or a user's personal identification number (“PIN”) to provide user authentication.
However, a disadvantage of the original CI specification is that it gave the potential for the decrypted digital content to be copied. This problem arises from the way in which the host and CAM interact. In use, the host sends encrypted data to the CAM. The CAM checks the user authentication and, assuming that the user is authenticated, it decrypts the access-controlled content. The CAM then sends the decrypted content back to the host over the CAM-host interface, which is generally a PCMCIA (Personal Computer Memory Card International Association) interface, though it is not limited to this interface—for example, a USB interface could be used. This connection from the CAM to the host represents a security weakness, in that the decrypted digital content can in principle be intercepted and unlawfully copied. This security weakness meant that some content providers preferred integrated devices, which have the host and CAM as a single unit, because this allowed them better security over the transfer of unencrypted data from the CAM to the host. However, this of course acted against the advantage associated with CI, relating to the potential interoperability of different CAMs and hosts.
The CI Plus specification was drafted to address these problems, by two main routes. CI Plus provides a secure interface between the CAM and the host, so that decrypted content data is not sent in clear form between the two devices. Also, CI Plus provides for the authentication of both the host and the CAM, rather than the CI technique of authenticating only the CAM.
The authentication system uses certificate hierarchy so that the host and the CAM must both have been issued certificates by an authority (such as CI Plus LLP).
The PCMCIA interface between a host and a CAM is protected by encrypting the decrypted content data before it is sent from the CAM to the host, and then decrypting it at the host. This encryption is separate to the access control encryption-decryption established by the content provider, and is specific to each particular CAM-host pair. Keys are exchanged between the CAM and host by the Diffie-Hellman key exchange technique. The keys are also cycled from time to time, so that even if a key was compromised, it would in any event be changed a few seconds later.
The CI Plus specification version 1.3 includes a so-called operator profile arrangement. Operator profiles allow the selection of a network information table (NIT) from a choice of different sources. The nature of the NIT will now be described.
The NIT carries network specific information, and may relate to a network encompassing more than one radio frequency (RF) channel, and more than one transport stream (TS). Information carried by the NIT can include radio frequencies or RF channel numbers applicable to that network, and a mapping between logical channel numbers (LCNs) and programme identifiers broadcast by the network. Logical channel numbers relate to the order in which television channels are presented to the user for selection at an individual host (by a channel selector or an electronic programme guide (EPG)) and are generally independent of the actual mechanism by which the television channels are transmitted. For example, in the United Kingdom the broadcast channel BBC1 is assigned to LCN 1 (in a terrestrial system) or 101 (in a satellite system). This assignment is entirely independent of the actual radio frequency by which the channel BBC1 is transmitted, and is also entirely independent of which packet identifiers (PIDs) the BBC1 transport stream is allocated for transmission. The NIT provides a mapping between the LCN and the transmission information such as RF channel or TS. To tune to a particular channel such as BBC1, a receiver may consult the NIT to establish which TS carries BBC1, then tune to the RF frequency carrying that TS, then filter packets from that TS with PID=0 to obtain the programme allocation table (PAT), then (once a channel selection is made) use information from the PAT to filter packets of the programme map table (PMT) to obtain the PIDs of so-called elementary streams corresponding to the desired channel (BBC1), then filter the packets to obtain the required PIDs to allow decoding of those elementary streams.
Previously, the NIT was delivered as part of the broadcast signal. The operator profile arrangement of CI Plus 1.3 allows two choices of delivery of the NIT, and in particular allows the potential delivery of a NIT via the CAM rather than reception as an over the air (OTA) delivery.
There are two operator profile settings in CI Plus 1.3:
profile_type=0: in this mode, the NIT is determined from the broadcast information; and
profile_type=1: in this mode, the CAM delivers a NIT to the host, and the host generates a LCN list dependent upon the NIT delivered by the CAM.
The CAM can generate the NIT from various sources. In principle, the NIT may be pre-programmed into the CAM. Or the CAM can receive the NIT as an OTA signal directed to that CAM or to CAMs provided by a particular network. Or the network can convey the NIT to the CAM by the low speed communications (LSC) mechanism provided by the CI Plus specification, using a secure separate connection such as an internet connection. The operator profile setting can also be provided by the CAM based on operator data from any of these sources.
In operation, the host queries the CAM to ask which operator profile is applicable. If the operator profile is type 0, then the host demultiplexes the NIT from the broadcast signal. If the operator profile is type 1, then the CAM provides the NIT to the host.